1. Field of the Invention
The present invention relates to an apparatus for image correction in a color television receiver, and particularly to a cathode ray tube control apparatus for automatically applying various corrections.
2. Description of the Prior Art
In a conventional video projector that projects an enlarged image to a screen using three projection tubes, each projecting one of the three primary colors, differences in the incidence angle of each projection tube to the screen (hereafter "convergence angle") can result in color shifting on the screen, blurred focus, deflection distortion, and changes in brightness. To compensate for these possible sources of image deterioration, an analog correction wave synchronized to the horizontal and vertical scanning periods is typically generated, and the amplitude and shape of this wave are appropriately adjusted. There are problems with the correction precision of this method, however. In addition, this is also a time-consuming adjustment because the adjustments are made manually with the technician visually confirming the adjustments on screen. A digital convergence apparatus (Japanese patent laid-open 59-8114) providing a high convergence precision adjustment, and a cathode ray tube control apparatus (electron beam deflection control apparatus) (Japanese patent laid-open 58-25042 and 58-24186, U.S. Pat. No. 4,456,853 to Robinder et al., U.S. Pat. No. 4,857,998 to Tsujihara et al. and U.S. Pat. No. 5,111,284 to Tsujihara et al.) for automatically correcting deflection distortion have been proposed as ways of correcting these problems.
FIG. 28 is a block diagram of a conventional cathode ray tube control apparatus with an automatic correction capability, and FIG. 29 is a screen image of the index phosphors in the corresponding cathode ray tube. As shown in FIGS. 28 and 29, the position of the electron beam from the index phosphor 6 coated on the shadow mask 43 surface of the cathode ray tube (CRT) 40 is detected by a detector 60. The detection signal output from the detector 60 is used by the process controller 66 to generate the signals for convergence correction and image geometry (geometric distortion) correction. The signal from the process controller 66 is input to the waveform generator 52. The waveform generator 52 thus generates the scanning waves driving the convergence yoke 44 and deflection yoke 46, and convergence and image geometry are automatically corrected. It is therefore possible as described above to automatically correct the electron beam position and thus control convergence and image geometry.
The problem with this conventional cathode ray tube control apparatus is that the index phosphor coating must be applied precisely perpendicular to the main scanning direction because the index phosphor comprises the vertical and hypotenuse legs of a right triangle where the vertical leg is perpendicular to the main scanning direction. This is a problem because the coating process requires a high precision coating technology. In addition, when measuring the position of the test signals 8, 9 emitted to the index phosphor, the measurement precision, and hence the correction precision, drop when the time measurement range between two index phosphors is very large, or the signal width varies because of the spot characteristics or aberrations in the test signal (beam). Finally, because the number of convergence correction points is determined by the number of index phosphors coated on the cathode ray tube, this conventional method cannot adapt to a variable number of correction points.